Yeah, there's not really a point of 'more than x number of death is intentional feeding'. I've seen people legitimately go 0/8/0 before. Occasionally you'll get in a game where one of the teams is simply outclasses, and they don't get a single kill. I've been on both the giving and receiving ends of this situation before.
On the contrary, I've seen people with positive records in the tribunal who I did find guilty of intentional feeding. These tend to be fairly obvious, because there is chat evidence of it. They'll say something like "I'm tired of carrying you people, gonna feed now".
So when I judge for these sorts of things, I take into account the whole picture. What are their items? How long was the game? How many deaths did they have? Kills and assists? What about CS? What were they behind in levels compared to the rest of the team? Is there chat evidence of it? How many people reported them? Did their opponents report them for it as well?
A good reason why you simply can't look at quantity of deaths alone was illustrated to me in a game the other day. We had a guy on our team pick jungle Rengar. Okay, that's fine. But he ran straight into mid and fed first blood to their mid before the game had really even gotten started. He then proceeded to leave the game. So at this point, it looks like he'll just get a leave, with an 0/1/0 record with 0 cs. However, at around the 19:00 mark, the guys reconnects to the game, presumably to avoid getting the actual leave mark on his record. He proceeded to just feed 3 more kills before it hit 20:00 and we were able to surrender.
So he would have ended the game 0/4/0, which is a believable record for simply having a bad game. But he also had under 10 cs at 20:00, and would have been pretty well underleveled.